J. Phys. Chem. C 2010, 114, 14441–14445
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Optically Controlled Phase Variation of TiO2 Nanoparticles Pier Carlo Ricci,*,† Alberto Casu,† Marcello Salis,† Riccardo Corpino,†,‡ and Alberto Anedda†,‡ Dipartimento di Fisica, UniVersita` di Cagliari, s.p. n°8 Km 0, 700 09042 Monserrato, Cagliari, Italy, and CGS, UniVersita` di Cagliari, s.p. n°8 Km 0, 700 09042 Monserrato, Cagliari, Italy ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: July 20, 2010
In this paper, we report the anatase-to-amorphous phase conversion of TiO2 nanoparticles by means of low intensity visible light excitation. A proper choice of power density of the incident radiation and of partial pressure of oxygen in the experimental room allows a local and precise control on the phase of TiO2 nanoparticles. The 2.54 eV light of an Ar+ laser was used to induce structural changes on the samples and to collect photoluminescence spectra, while the 1.96 eV light of a He-Ne laser was utilized for in situ Raman measurements to give immediate proof of the crystallographic structure of the samples. In opposition to other light induced processes, based on the temperature effect, the phase conversion is due to surface modifications by a proper depletion of adsorbed oxygen on the crystal surface. Introduction
Experimental Section
Nanosized titania (TiO2) crystals are promising materials for their appreciable characteristics, such as low production cost, nontoxicity, and high stability against high energy irradiation; subsequently, a great deal of effort was devoted to the study of TiO2 in photocatalyst, dye-sensitized, and photovoltaic applications.1,2 The most common crystal structures of TiO2 are anatase, rutile, and brookite, the latter being known as a high-pressure phase. Anatase converts to rutile at temperatures between 400 and 1200 °C, but different parameters such as grain size, impurities, and material processing influence the effective rate of the polymorphic reaction.1,3-5 Recently, it was reported that room-temperature phase conversion of anatase to rutile TiO2 can be achieved by doping the TiO2 nanoparticles with transition metal ions.6 These transformations are very important both from a scientific as well as a technological point of view, as they can be useful in fields such as the photochemistry of TiO2 surfaces, which is deeply correlated with the band gap, the electronic structure, and the role of bulk and superficial defects.7,8 In this paper, we report evidence of optically assisted phase transformation of TiO2 nanocrystals from anatase to amorphous. In opposition to other light induced processes (based on temperature effect),9,10 in the present case, the reason for phase conversion must be ascribed to surface modifications by a proper depletion of adsorbed oxygen on the crystal surface. The phase conversion at room temperature is induced by low intensity visible light excitation, and it brings out the key role of oxygen vacancy defects and chamber atmosphere on high surface TiO2 samples. “Low excitation intensity” should be intended as not capable of producing a significant heat contribution. The analysis of in situ Raman spectra gives immediate proof of the crystallographic structure of the samples and was utilized as a probe to study the effect of visible and intrinsic excitation of bulk TiO2 as a function of the chamber atmosphere.
TiO2 powders were prepared by the sol-gel technique, with the preliminary formation of an amorphous solid (xerogel) and a subsequent thermal treatment of the final powders at 300 °C for 6 h under oxygen stream. The analysis of X-ray diffraction patterns reveals that nanocrystals were formed mainly in the anatase phase with a mean nanoparticle size of 6.4 nm. Details on the preparation and final structure can be found elsewhere.11 Two excitation energies were used during the experiment: the 1.96 eV light from a He-Ne laser with energy density lower than 0.1 W/cm2, intended to measure Raman spectra without altering the sample structure, and the 2.54 eV light of an Ar+ laser to induce structural changes on the analyzed samples. The latter was also used for luminescence measurements with a power density as low as 0.1 W/cm2. In order to study the structural change, experiments were performed with different excitation power densities of the 2.54 eV source, starting from 0.1 W/cm2 and rising with steps of 0.5 W/cm2. Raman and luminescence spectra were collected after visible excitation. The effective local temperature was checked from the ratio of the Stokes/anti-Stokes Raman bands of the 144 cm-1 peak of the anatase phase for a power density of 10 W/cm2, which is well beyond the one utilized in the experiment, but no significant increase in the local temperature was observed. No phase change was revealed in the sample structure with red light as the sole excitation source and vacuum conditions well beyond the ones considered in the experiment (about 1.33 × 10-4 Pa). Raman analysis of the as-grown powder in room conditions reveals the typical spectrum of the anatase phase, while the 2.54 eV excitation beam induces a broad and weak luminescence centered around 2 eV, ascribed to the luminescent recombination at the oxygen vacancy sites.12
* To whom correspondence should be addressed. E-mail: carlo.ricci@ dsf.unica.it. Phone: +39 0706754755. Fax: +39 070510171. † Dipartimento di Fisica. ‡ CGS.
Results and Discussion Figure 1a reports the Raman spectra recorded after 20 s of visible light excitation for different excitation power densities (as labeled in Figure 1) with a chamber pressure of 4.66 Pa. Consequently, the PL signal intensity (Figure 1b) changes after visible illumination, indicating a structured recombination process.
10.1021/jp105091d 2010 American Chemical Society Published on Web 08/12/2010
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Figure 1. Raman (a) and photoluminescence (b) spectra of TiO2 nanoparticles after illumination with a 2.54 eV laser beam and power density as indicated in the legend in vacuum conditions. The spectra were acquired after excitation by a low intensity laser (0.1 W/cm2) at 1.96 and 2.54 eV for Raman and photoluminescence measurements, respectively.
Figure 2. Optical (left) and Raman (right) imaging of TiO2 nanopowders after visible light illumination in vacuum conditions. The varying color between orange and white indicates the local structure of the sample ranging from amorphous (orange) to anatase (white), with the pale orange region being the one of degraded anatase.
A sudden phase variation is evidenced in the spectrum recorded after excitation with 4 W/cm2. The absence of the typical Raman anatase peaks reveals a degradation of the initial phase and suggests the formation of an amorphous one. This indication is confirmed by the increase of the PL band previously ascribed to the presence of oxygen vacancy defects.12 Optical and Raman imaging of the sample after excitation in vacuum conditions is reported in Figure 2 to better show the local effect of visible light illumination. The irradiated zone appears optically orange, while the part of the sample kept in the dark remains white. Only the irradiated zone of the sample presents a Raman spectrum different from the anatase phase, while the border of the illuminated region shows a Raman spectrum of the deteriorated anatase phase. It is worth noting that an opposite behavior can be observed in air: Raman peaks of anatase remain unchanged or at least more settled as the power density increases, while the photoluminescence intensity quickly decreases (Figure 3) with irradiation time. Moreover, the trend observed on anatase Raman peaks as well as the luminescence band is partially compensated by keeping the sample in the dark at room conditions for several hours. Similar results were previously observed in TiO2 nanocrystals with a mean diameter of 7.9 nm for excitation at 2.60 eV in air, and the PL intensity decay was assigned to light induced change in the surface electronic structure.12
Figure 3. Variations in the integrated area of the photoluminescence band vs the irradiation time of TiO2 nanoparticles after illumination with a 2.54 eV laser beam in air. The insets show the Raman spectra acquired with the minimum and maximum irradiation times, respectively. The spectra were acquired after excitation by a low intensity laser (0.1 W/cm2) at 1.96 eV (Raman) and 2.54 eV (photoluminescence).
As previously shown in Figures 1 and 3, evolution in the behavior of anatase nanoparticles after visible light illumination is strongly influenced by the atmosphere in the experimental chamber. Recent and independent investigations show that, depending on the initial surface conditions and on the oxygen partial pressure in the experimental chamber, crystal excitation with
Phase Variation of TiO2 Nanoparticles
Figure 4. Schemes of energy levels and illustrations showing the reaction of TiO2 nanoparticles to irradiation in air conditions (A) and vacuum conditions (B). Both of the schemes depict the F-type color centers as a black-colored level on the bottom right corner of the scheme, the titanium atoms as a cyan(3+)/blue(4+) level on the top right corner, and the oxygen ions as a gray level on the left side of the scheme. For clarity purposes only, the 5-fold titanium-coordinated oxygen is depicted in the scheme. The illustrations depict the final conditions on the surface of TiO2 nanoparticles in both conditions.
suitable light can increase the net rate of either absorption12 or desorption19 of molecular oxygen. The increase of surface activity under light excitation can be attributed to the increase of electron and hole centers available for capture and recombination processes at specific vacancy sites. The excitation energy in the experimental setup is lower than the optical band gap of the anatase nanosized particles, so absorption of visible light should be ascribed to the presence of charged color centers.13 The nature of these color centers is strictly related to oxygen vacancies14 generated by a proper choice of growing parameters like temperature and chamber atmosphere or controlled by doping elements.15,16 According to Serpone,17 the loss of an O atom in a metal oxide leaves behind an electron pair trapped in VO giving rise to an F center, while a positively charged F+ center is equivalent to a single electron associated to the O vacancy:17
-O2- - f O + VO + 2eVO + 2e- f F VO + e- f F+ Thus, the color centers associated with oxygen vancancies are neutral F and positive charged F+ and F+2 centers.18 The energy scheme of the anatase nanoparticles should follow the diagram of energy levels illustrated in Figure 4, so that the behavior of the sample under intrinsic excitation with different atmospheric conditions can be discussed as follows: in an oxygen-rich atmosphere, the O2 molecules act as photoexcited electron scavengers, thus collecting the electrons raised to the conduction band from F-type color centers (Figure 4A).13,19,20 The O2 molecules can be adsorbed on the TiO2 surface at least in two specific sites: from a five-coordinated Ti3+ site, generating O2- by one electron transfer from Ti3+ to the adsorbed O2 molecule (Figure 4A, blue Ti balls and white O balls), or a vacancy site, where two Ti3+ ions “close” each other with a bridge provided by the captured O2 molecule, thus forming a diamagnetic peroxide ion O22- between the two Ti3+ ions (Figure 4A, blue Ti balls and purple O balls). Both sites compensate oxygen vacancies, and a continuous visible illumination facilitates O2 adsorption and compensation of defects
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14443 present at the TiO2 surface from the initial conditions. The decay in PL intensity is a direct indication of the compensation of surface defects due to adsorption of O2 molecules during blue illumination, as already observed after band to band excitation.20 In our opinion, the visible excited luminescence is related to electron-hole recombinations taking place between nearest Ti3+ and F+ centers. In accordance with this hypothesis, in a recent theoretical work based on DFT in local density approximation, the model featuring the excess of one electron trapped in Ti3+ sites is very close in energy with partially or highly delocalized DFT calculated solutions where the extra charge is distributed over several Ti ions, thus suggesting the important statistic role of temperature and time.21 This fact may explain the reactivation of PL peaks after keeping the sample in dark conditions for several hours, already observed in a previous work.13 In vacuum conditions (Figure 4B), the effect of promoting the photoexcited electron to the conduction band is the deadsorption of O2 molecules. Starting from the stable situation in which the O2- are adsorbed on the oxygen vacancy site of the TiO2 surface (in a 5-fold bond or as a bridge between two Ti4+) and from the experimental assumption that no further oxygen molecules are available in the experimental chamber, illumination with visible light causes electrons from the color centers to be excited to the conduction band. The photoexcited electrons in the conduction band will be captured by Ti4+ at the surface in correspondence to oxygen vacancy sites, while O2- and O22leave their electrons to the F-type color centers, thus allowing the deadsorption of O2 molecules. In truth, photodesorption kinetics of O2 molecules in an ultrahigh vacuum chamber was studied on TiO2 rutile samples under extrinsic excitation in recent investigations.20 The photodesorption is caused by recombination of electrons trapped in O2- molecules with holes in the valence band. Although our experiment is performed with intrinsic excitation, the mechanism remains very similar and brings to the formation of a high superficial density of Ti3+. Actually, the well-established reaction scheme, known to occur during UV irradiation in anatase TiO222
TiO2 + hν f TiO2(h+ + e-) is still valid, but now the holes are localized in the F+ centers; conduction band electrons can be trapped in Ti4+ ions in the bulk or at the surface:
Ti4+ + e- f Ti3+ The kinetics of surface processes involving oxygen exchange can be accounted by the rate equation
dnO2 dt
) γApnS - γDnO2m
(1)
where nO2 stands for the density of adsorbed O2 molecules, m and nS for the densities of trapping hole and electron centers, respectively, p for the gas pressure, and γA and γD for probability factors of O2 capture and electron-hole recombination, respectively. In eq 1, we indicate the total air pressure instead of the oxygen partial pressure. This can be admitted in our case where different air pressure conditions with fixed molar composition are considered. Densities of centers change with time and are dependent on the intensity of exciting light. The
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Ricci et al. dependent only on light intensity and on temperature. Thus, the isothermal process of oxygen exchange from pressure P1 to pressure P2 can be described by24
g(I2, T) - g(I1, T) + T∆Sc ) RT ln
Figure 5. Chamber pressure vs laser power density (2.54 eV) graph showing the conditions of achieved amorphization of TiO2 nanoparticles irradiated in vacuum conditions and the best fitting curve for the chosen kinetic and thermodynamic model (also shown). The experimental error bars are below the dimension of the graphical points.
sign of the right member in eq 1 establishes how a surface attains its stationary state: desorption requires that
p
